1. Field of the Invention
This invention relates to the field of geophysical prospecting and, more particularly, to a method for generating seismic vibrator data using overlapping sweeps.
2. Background of the Art
In the oil and gas industry, geophysical prospecting techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon deposits. Generally, a seismic energy source is used to generate a seismic signal that propagates into the earth and is at least partially reflected by subsurface seismic reflectors (i.e., interfaces between underground formations having different acoustic impedances). The reflections are recorded by seismic detectors located at or near the surface of the earth, in a body of water, or at known depths in boreholes, and the resulting seismic data may be processed to yield information relating to the location of the subsurface reflectors and the physical properties of the subsurface formations.
One type of geophysical prospecting utilizes an impulsive energy source, such as dynamite or a marine air gun, to generate the seismic signal. With an impulsive energy source, a large amount of energy is injected into the earth in a very short period of time. Accordingly, the resulting data generally have a relatively high signal-to-noise ratio, which facilitates subsequent data processing operations. On the other hand, use of an impulsive energy source can pose certain safety and environmental concerns.
Since the late 1950s and early 1960s, a new type of geophysical prospecting, generally known as xe2x80x9cVIBROSEISxe2x80x9d(copyright) prospecting, has been used. Vibroseis prospecting employs a land or marine seismic vibrator as the energy source. In contrast to an impulsive energy source, a seismic vibrator imparts a signal into the earth having a much lower energy level, but for a considerably longer period of time.
The seismic signal generated by a seismic vibrator is a controlled wavetrain (i.e., a sweep) which is applied to the surface of the earth or in the body of water or in a borehole. Typically, a sweep is a sinusoidal vibration of continuously varying frequency, increasing or decreasing monotonically within a given frequency range, which is applied during a sweep period lasting from 2 to 20 seconds or even more. The frequency may vary linearly or nonlinearly with time. Also, the frequency may begin low and increase with time (upsweep), or it may begin high and gradually decrease (downsweep).
The seismic data recorded during Vibroseis prospecting (hereinafter referred to as xe2x80x9cvibrator dataxe2x80x9d) are composite signals, each consisting of many long, reflected wavetrains superimposed upon one another. Since these composite signals are typically many times longer than the interval between reflections, it is not possible to distinguish individual reflections from the recorded signal. However, when the seismic vibrator data is cross-correlated with the sweep signal (also known as the xe2x80x9creference signalxe2x80x9d), the resulting correlated data approximates the data that would have been recorded if the source had been an impulsive energy source.
The amount of energy injected into the earth during a conventional vibrator sweep is governed by the size of the vibrator and the duration of the sweep. Given current practical limitations on both vibrator size and sweep duration, it is usually necessary to generate several sweeps at each source point. Each sweep is typically followed by a listen-time during which the vibrator is not sweeping, but reflection energy is still being received by the seismic detectors. Data resulting from each sweep are then cross-correlated with the reference signal for that sweep, and the resulting individual data traces are summed or xe2x80x9cstackedxe2x80x9d to obtain the final composite data trace for the source point. A significant portion of the time required for each source point is associated with the listen time between sweeps. Obviously, the efficiency of Vibroseis prospecting could be significantly improved by eliminating part or all of this listen time.
Another problem with conventional Vibroseis prospecting results from the fact that vibrators generate harmonic distortion as a result of nonlinear effects in the vibrator hydraulics and the ground""s nonlinear reaction to the force exerted by the vibrator base plate, with the second and third harmonics accounting for most of the distortion. These harmonics are present in the recorded data and lead to trains of correlated noise, known as harmonic ghosts, in the correlated data. These harmonic ghosts are particularly troublesome in the case of downsweeps where they occur after the main correlation peak (i.e., positive lag times) and, therefore, can interfere with later, hence weaker, reflections. In the case of upsweeps, harmonic ghosts are somewhat less troublesome because they precede the main correlation peak (i.e., negative lag times). Nevertheless, harmonic ghosts can cause difficulties in processing and interpreting data from upsweeps as well as from downsweeps.
U.S. Pat. No. 5,410,517 issued to Andersen discloses a method for cascading or linking vibrator sweeps together to form a cascaded sweep sequence and optionally eliminating the listen period between successive sweeps. The initial phase angle of each individual sweep segment within a sweep sequence is progressively rotated by a constant phase increment of about 360/N degrees, where N is the number of sweep segments within the sweep sequence. Either the correlation reference sequence or the vibrator sweep sequence, but not both, contains an additional sweep segment positioned and phased so as to substantially suppress harmonic ghosts during correlation. When the additional sweep segment is included at the end of the vibrator sweep sequence, it increases the total acquisition time. If the correlation reference sequence includes the additional sweep segment, it complicates the processing in that the additional sweep segment has to be input at negative time giving a nonstandard correlation operator.
Rozemond discloses a method of seismic acquisition using multiple vibrators using the so-called xe2x80x9cslip-sweepxe2x80x9d method. The method consists of a vibrator (or a vibrator group) sweeping without waiting for the previous vibrator""s sweep to terminate. Correlation, which acts as a time-frequency filter, then extracts the individual records. A significant reduction in overall acquisition time is obtained. This is more efficient than the cascaded sweep since there is no need to wait for the end of a sweep before starting the next sweep. The reduction in overall acquisition time comes at the cost of increased harmonic distortion since the harmonics from the second sweep will correlate with the primary signals of the first sweep.
There is a need for an invention that acquires data with increased efficiency by using overlapping sweeps while providing some measure of protection against harmonics. The present invention satisfies this need.
The present invention is a method for increasing the efficiency of Vibroseis data acquisition using overlapping sweep signals while providing some protection against harmonic distortion. Two identical pairs of sweep segments are used with a possible overlap in time between the pairs. Each pair of sweep segments includes an earlier low frequency sweep and a later high frequency sweep, the individual sweeps having substantially no overlap in frequency except for tapering. The high frequency sweep in each pair starts before the end of the low frequency sweep with the start time selected so that the harmonics from the low frequency sweep fall outside the data window associated with the high frequency sweep. As a result of this, when the recorded signal from the two pairs of sweep segments is separately correlated with the low frequency sweep and the high frequency sweep, individual portions of the desired data are recovered with the harmonic distortion separable from the desired data. The individual portions of the desired data are then spliced to give a broadband response of the earth.
In an alternate embodiment of the invention, reference signals such as groundforce signals, signals from the motion of the vibrator base plate or signals from the motion of the vibrator reaction mass are measured. Filters are derived relating the reference signals to a desired short duration wavelet. The derived filters are applied to the recorded data and the resulting filtered data sets are combined to give a broadband response of the earth.